1. Field of the Invention
This invention relates to a toner used in electrophotography, electrostatic recording, magnetic recording and toner jet recording.
2. Related Background Art
Conventionally, electrophotography is a process in which an image is obtained by forming an electrostatic latent image on a photosensitive member by various means, developing the latent image by the use of a toner to form a toner image on the photosensitive member, transferring the toner image to a transfer material such as paper as occasion calls, and then fixing the toner image to the transfer material by the action of heat, pressure, heat-and-pressure, solvent vapor or the like.
As the step of fixing toner images, it has been put forward to use a pressure contact heating method making use of a heating roller (hereinafter “heating roller fixing method”), a heat fixing method in which toner images are fixed bringing a fixing sheet to which toner images are to be fixed into close contact with a heating element through a fixing film (hereinafter “film fixing method”), and so forth.
In the heating roller fixing method or the film fixing method, toner images held on the fixing sheet are made to pass the surface of the heating roller or fixing film while bringing the former into contact with the latter under application of pressure by means of a pressure member kept in touch with the latter. In this fixing method, since the surface of the heating roller or fixing film and the toner images held on the fixing sheet come into contact with each other under application of pressure, the heat efficiency in fusing the toner images onto the sheet is so greatly high as to enable performance of rapid and good fixing. In particular, the film fixing method is greatly effective in energy saving, and is expected to be also effective in that, e.g., time can be short which is taken after the switch of an electrophotographic apparatus is turned on and until printing on the first sheet is completed.
Electrophotographic apparatus are variously demanded to be achievable of high image quality, compact and light weight, high-speed high-productivity, energy saving and so forth. In particular, especially in the fixing step, it is an important technical subject to develop systems and materials which can achieve much higher speed, energy saving and high reliability.
However, in order to resolve such a subject in the heating roller fixing method or film fixing method, it is essential, in particular, to vastly improve fixing performance of toners. More specifically, it is necessary to improve the performance of being sufficiently fixable to the fixing sheet at a lower temperature (hereinafter “low-temperature fixing performance”) and to improve the performance of being able to prevent offset which is a phenomenon in which contamination due to toner having stuck to the surface of the heating roller or film causes contamination of a next fixing sheet (hereinafter “anti-offset performance”). Also, as performance tending to come into the relation of a trade-off for the improvement in the low-temperature fixing performance, there may be given the performance of keeping a phenomenon from occurring in which the toner comes to agglomerate or fuse during long-term storage (hereinafter “anti-blocking performance”) and the performance of keeping any faulty images from coming about in continuous printing on a large number of sheets (hereinafter “development stabilizing performance”).
As full-color electrophotographic apparatus have become popular, it has also become required to further improve image quality level. More specifically, what is required is the performance of keeping the toner from soaking into paper so much as to narrow its image color range (hereinafter “anti-soaking performance”). This soaking tends to occur as a lowering of image quality level that is due to heating non-uniformity coming about in the direction of progress of the fixing step between the first half and the second (latter) half of fixing, or as a lowering of image quality level that is due to heating non-uniformity between the first sheet and the 10th sheet when images are reproduced at a high speed. Also, in color toners, images having a broad image color range (hereinafter “color ranging performance”) are demanded, where images having a higher image chroma or images having higher image brightness are demanded even when image densities are equal to one another. Such color ranging performance of the toner is concerned with (1) the color developing performance a colorant contained in a toner has, (2) the state of presence of the colorant in a toner, (3) the transparency of a binder resin and other components contained in a toner, (4) the surface state of toner layers formed by the fixing of toner images onto a transfer material, and so forth. In particular, it is important to form the toner layers on the transfer material in a more uniform surface state.
In toners used in heat-and-pressure fixing, a toner having a capsule structure is available as a toner which aims to achieve both the low-temperature fixing performance and the anti-blocking performance (see Japanese Patent Application Laid-open No. H06-130713). This toner is one in which inner core layers having a low glass transition point (Tg) is covered with outer shell layers having a high Tg so that any low-Tg material contained in the interiors of toner particles may be kept from exuding, so as to achieve both the low-temperature fixing performance and the anti-blocking performance or development stabilizing performance. Also, as a method of afterwards forming the outer shell layers covering the surfaces of inner core layers of toner particles, a toner is proposed the particles of which are provided with intermediate layers having a chargeability that is reverse to the chargeability of the inner core layers and outer shell layers (see Japanese Patent Application Laid-open No. 2003-091093). This toner is one in which high-Tg and high-molecular weight resin particles or inorganic particles are introduced into the intermediate layers to make the outer shell layers able to gain their weight, so as to aim to improve the anti-blocking performance and the development stabilizing performance. However, it is sought to make more improvement in low-temperature fixing performance and to make image quality higher.
For the purpose of preventing a phenomenon that toner images formed on a transfer material stain the other transfer materials, a toner is proposed in which a storage elastic modulus G′ at 30° C. and a loss tangent (tan δ) at 60° C. in a dynamic viscoelasticity test have been controlled (see Japanese Patent Application Laid-open No. 2002-287425). In this toner, however, substantially the value of tan δ at 60° C. is 0.7 or more and the value of G′ at 30° C. is 2×108 Pa or more. A toner is also proposed which has a minimal value and a maximal value at 70° C. or more to less than 110° C. in a loss tangent (tan δ) curve in a dynamic viscoelasticity test and in which a loss elastic modulus G″ at 140° C. has been controlled (see Japanese Patent Application Laid-open No. 2006-235615). However, it is sought to make more improvement in low-temperature fixing performance and to make image quality higher.
As a toner which aims to achieve both low-temperature fixing performance and glossiness uniformity, a toner is available in which the range of change in loss elastic modulus G″ in the temperature region of from 60° C. to 95° C. has been controlled (see Japanese Patent Application Laid-open No. 2006-091168). However, the toner has had an insufficient anti-soaking performance because of its great change in viscosity in that temperature region.